1. Field of the Invention
This invention describes a method and apparatus applicable to heat exchange devices wherewith high heat transfer efficiency can be achieved with inconsiderable power requirements necessary to overcome the fluid frictional resistances directly related to heat transfer. More particularly, this invention relates to high heat flux rotary type heat exchangers having flexibility to accommodate two or more streams in countercurrent, cocurrent, and series arrangements, and having a simply constructed, discretionary means to substantially or partially recover the mechanical energies transferred to the fluid streams during heat exchange.
2. Description of the Prior Art
Conventional heat exchangers both stationary and rotary consume power during the exchange of heat between fluid streams. With stationary heat exchangers the power is irreversibly lost, that is, the kinetic energy imparted to the fluid to cause flow along the heat conducting surface cannot be conveniently recovered. Contrastly many types of rotary heat exchangers offer the potential for practical power recovery, since kinetic energy is commonly added to the fluid. However, this inherent advantage has not been utilized except in specialized applications which routinely require heat transfer in conjunction with pumping or fan action. Thus rotary exchangers have remained well known in the art but little used in applications.
Three basic types of rotary heat exchange devices are available to the prior art. The first and perhaps most widely known and applied is the axial flow type. Most recent vintage art is applied to improvements in materials, fluid seals, and physical stability under high temperature environments. The second type includes the radial flow devices, which are characterized by less developed art and more limited application. Recent art teaches new configurations advantageous to novel applications and performance improvements generally pertaining to increased surface area per volume occupied by the device. The third type commonly involves binary or infrequently tertiary combinations of axial, circumferential, or radial flow arrangements, having art and applications similar to the above radial flow devices.
In axial heat exchangers gases pass axially through a cylindrically shaped core. Hot gas flows through one portion while cold gas flows countercurrently through the other. A longitudinal seal separates the hot portion from the cold portion preventing leakage between hot and cold gases. By rotating the core, heat is continually transported from the hot gas to the cold gas via the heat conducting capacitance of the core material. The cores of axial heat exchangers are made of heat resistant alloy metals or ceramic materials in a closely spaced matrix form. The matrix design seeks to maximize heat transfer surface area while minimizing flowing pressure losses imposed by gas movement through the matrix. The matrix design further seeks to minimize the quantities of gases transported from one core portion to another by its rotation, thereby implicating core length, rotational speed, and flowing pressure losses into the design objectives. These and other characteristics are well known. U.S. Pat. No. 4,331,198 describes this art and discloses improvements on inherent problems, while U.S. Pat. No. 4,174,748 presents a novel, radially configured rotary core.
In radial type rotary heat exchangers, the heat transfer surfaces separating two fluid streams are provided by circular discs affixed to a rotating shaft. Commonly, as disclosed in U.S. Pat. No. 4,431,048, a first large volume rate stream flows radially outward along the disc's external surfaces from a central, axially aligned cylindrical ductway. The second, relatively small volume rate stream flows radially outward then radially inward inside internal passages formed within the discs. Normally, radial heat exchanges are used only in applications in which the first stream requires the pumping or fan action imparted by disc rotation.
In mixed flow rotary heat exchangers, several combinations are available to the art. Usually, applications are restricted to pumping or fan action situations. The radial/axial configuration, such as U.S. Pat. No. 3,989,101, generally cites flow of a first high volume rate, low pressure stream from a central distributor duct into radially outward disposed heat exchange passages. The low volume rate, frequently pressurized second stream flows within axially aligned conduits. A series of plates embodies the radial passages and supports the axial conduits. Another common mixed flow configuration involves the radial/circumferential alternative as taught in U.S. Pat. No. 4,073,338. This art is similar to the above radial/axial configuration except that the channels for the low volume rate stream are annular circumferential tubes or hollow discs. In still other citations, less known configurations are presented. U.S. Pat. No. 4,074,751 teaches an axial/circumferential arrangement comprising circumferential tubes in combination with axial flow area formed by the rotor enclosure. Adjunctly, U.S. Pat. No. 3,424,234 teaches a tertiary flow arrangement involving radial flow of a large volume rate, low pressure stream and a circumferential/axial flow complexity for the small volume rate stream.
In these known rotary exchangers three common problems arise which this invention proposes resolving.
The first problem concerns the heat transfer rate between material surfaces and the fluid streams. In most types of rotary exchangers, the convective coefficient is undesirably low. For example, U.S. Pat. No. 4,331,198 cites nominal axial type convective coefficients ranging from 3.2 to 6.8 BTU/HrFt.sup.2 .degree.F. as common for the hot and cold gases. In further complicity laboratory researchers at Michigan Technological University report that heat transfer to a fluid in radial flow between two corotating discs is only slightly affected by rotation; that is, heat transfer from stationary discs is essentially the same as that observed from discs rotating at moderately high speeds (200 to 600 RPM test conditions).
The second problem concerns the pumping or fan action imparted to the fluid streams by the radially oriented rotative members. In many applications this action is undesirable and a burden on the mechanical energy required to maintain rotation.
The third problem concerns the limited flexibility of known rotary heat exchangers to continuously accommodate more than two streams in various combinations of countercurrent, cocurrent, and series flow arrangements. In all known prior art citations, a two stream configuration was disclosed. It is a reasonable assumption that the previous two problems are important factors having influence over the limited flexibility.